Among the known methods for providing transfer of a thin layer, the method known by the name of “Smart Cut™” has become particularly significant. This method uses ion implantation in a substrate, by bombarding one of the faces of said substrate using, typically, a flux of hydrogen, helium and/or rare gas ions. This ion bombardment results in a concentration of these ions in a zone located at a given depth within the substrate under the bombardment surface; a second substrate is typically joined to the first substrate along the bombardment surface (this second substrate can have been formed previously and is bonded to the first substrate, typically by molecular bonding; as a variant, it can be formed directly on said bombardment surface, for example by deposition), then thermal and/or mechanical-chemical conditions are applied to the assembly which, taking into account the implantation conditions and subsequent treatments, cause separation of the thin layer (situated between the bombardment surface and the concentration zone) with respect to the rest of the first substrate; the method can then be repeated, taking as starting substrate, the rest of the aforementioned substrate, after separation of the thin layer (a plurality of thin layers can thus be formed in the thickness of the same substrate). This method is in particular described in U.S. Pat. No. 5,374,564 (Bruel) and—U.S. Pat. No. 6,020,252 (Asper et al.).
Several variants have subsequently been proposed, consisting for example of implanting, by bombardment, ions which by themselves do not form gas molecules, but which can promote the formation of gas molecules from elements of the first substrate. For example as disclosed by PCT published application No. WO2007/110515 to Tauzin.
Moreover, use of other techniques for introducing ions, such as plasma immersion or high-temperature diffusion, has been envisaged.
A thin layer thus transferred serves in practice as a support for microcomponents (microelectronic, micro-mechanical, micro-acoustic, micro-optical and the like), optionally formed wholly or partly before separation of said thin layer, so that it is important that the thin layer obtained by such a transfer has optimum microcrystalline, electrical, mechanical, properties and the like; it is moreover important that the operations of implantation, of bonding and of separation are compatible with the stages of formation of the components intended to be carried by this thin layer, when certain of these stages of formation are carried out prior to separation. In particular, it is generally considered desirable not to exceed certain temperature thresholds, for example of the order of 400° C., or even lower, and to manage to introduce the ions into the concentration layer without excessively degrading the thin layer that is to be transferred. In practice, bonding of the future thin layer to the substrate to which it is to be transferred involves thermal treatments, either for consolidating molecular bonding with this destination substrate, or for depositing a thick film that is to form said destination substrate; moreover, ion implantation by bombardment, which may seem preferable to techniques of plasma immersion or high-temperature diffusion, causes perturbations in the microcrystalline lattice of the thin layer, which may, under certain conditions, degrade the mechanical or electrical properties of the thin layer.
In practice, layer transfer by ion implantation is limited to layer thicknesses equivalent to the depth attainable by the implanting equipment. Unless implanters that are very powerful and therefore very expensive are used, this maximum thickness is of the order of a micrometer.
This explains why it is advantageous for a person skilled in the art to have several transfer techniques at his disposal, so as to have a choice for a particular case.